1. Field of the Invention
The present invention is related to a method of and an apparatus for retouching an image to reform a contour in the image.
2. Description of the Prior Art
In reproducing an original image, a reading apparatus such as an image scanner firstly scans and reads a color original or a monochrome original to obtain an image signal. The image signal is subjected to digital image processing, and a reproduced image represented by the processed image signal is displayed on a display means such as a CRT. During this procedure, a noise component is added to the image signal due to so-called sampling processing, detail enhancement processing, which are executed in a reading apparatus, an image processing apparatus or a recording apparatus, and defects are added to the original.
The noise component makes contours, lines and curves in an image uneven and broken. The contour denotes a portion of an image where a large difference of density exists. This phenomenon caused by the noise component is generally called "JAG" in the field.
In order to prevent the JAG, some techniques are developed as described in Japanese Patent Publication Gazette No. 63-49426, Japanese Patent Laying Open Gazette No. 61-95669, and the like.
The technique disclosed in Japanese Patent Publication Gazette No. 63-49426 is based on the following smoothing processing: a weighted signal is obtained by weighting density signals about a pixel and its adjacent pixels with respective weighting factors and by adding them together, and the weighted signal is compared with a prescribed threshold value, whereby an area of medium density is formed at a contour portion.
The technique disclosed in Japanese Patent Laying Open Gazette No. 61-95669 first obtains a differential signal between a sharp signal and an unsharp masking signal. The differential signal is compared with a prescribed slicing levels of plus and minus, whereby a ratio of the magnitude of detail enhancement processing to that of smoothing processing is adjusted.
However, the above stated techniques pay no regard to the directions of the detail enhancement and the smoothing. Therefore, the reproduced image obtained thereby seems dimmer all over than the original image. Further, since the entire image area is processed in these techniques, the processing is performed even in the areas where the JAG does not exist if the threshold value (or a slicing level) is incorrectly set, whereby the reproduced image seems unnatural.
There is another technique where the magnitude of detail enhancement is maintained comparatively small in order to prevent the JAG. According to this technique, however, the entire reproduced image seems dimmer, and the JAG is not well prevented.